The present invention relates to a short stroke linear motor which is designed for a highly dynamic position control for very small stroke movements. Short stroke linear motors of this kind are used in machine tools for so-called eccentric turning such as is necessary, for example, for machining pistons of internal combustion engines. In eccentric turning, a very small (<<1 mm) but specific deviation from the circular shape is aimed for. This is achieved by an oscillating movement of the cutting tool synchronized with the rotational speed as the workpiece turns. To achieve high dynamics when eccentric turning, a linear motor with a high intrinsic acceleration value is required. According to the equation amotor=Fmotor/mmotor (N/kg), this means a motor force which is as high as possible with minimum motor mass.
As a rule, three-strand synchronous linear motors are used as standard as controlled direct drives for the stroke movement of the cutting tool in machine tools for eccentric turning. A three-strand synchronous linear motor of this kind is reproduced in longitudinal section in FIG. 1. In order that the moved mass remains as small as possible, the cutting tool is moved by the secondary part 1 of the linear motor, as the secondary part 1 is lighter than the primary part 2 of the linear motor. The secondary part typically consists of a steel carrier plate 3 on which the permanent magnets 4 are fixed. These are aligned alternately from the secondary part 3 to the primary part 2 and vice versa.
In order to further increase the motor dynamics, the mass of the moved secondary part is minimized by optimizing the design with regard to geometry and stiffness. This optimization, with which a sufficiently small mass and high stiffness of the secondary part is achieved, often results in a technically elaborate design of the secondary part 1. In particular, complex machining is then required in many instances, and expensive materials must be used.
In its primary part 2, the standard three-phase synchronous linear motor reproduced in FIG. 1 has numerous teeth 5 which are aligned towards the secondary part 1 and are in each case encompassed by a coil 6. The linear motor specifically shown in FIG. 1 is distinguished by the following data:
Number_teeth: Nz=12
Number_active_magnets: NM=13
Length of primary part: LFe=Nz*τz 
Tooth pitch: τz 
Magnet pitch: τM 
Pitch ratio: τz≠τM 
Number of strands: m=3 (symmetrical)
Motor terminals: 3 (U, V, W)
max. operating force: 100% Fnenn 
min. operating force: 100% Fnenn 
The magnets covered by the length of the primary part (LFe=Nz*τz) are designated here as active magnets (number: NM).
For multi-phase (m>1) linear motors, and therefore also for the three-phase linear motor in wound tooth technology shown in FIG. 1, the following conditions must be maintained:τz≠τM LFe=Nz·τM±n≠τM; n=1,2,3 . . .
The three-phase synchronous linear motor generates a constant force over the whole range of travel, which is achieved by the sinusoidal current characteristic. Depending on position, only one motor strand (e.g. Strand U) is ever in the optimum position to produce maximum force, and at this moment the other two motor strands (e.g. Strands V and W) are in a different pole position with respect to the magnets and do not utilize their current in an optimum manner. In short stroke applications (e.g. eccentric turning), the maximum range of travel smax is relatively small: Smax≈1 . . . 2 mm or smax<<τm. This means that the three-phase linear motor practically works at one operating point and does not provide the optimally achievable force, as not all motor strands are in the optimum pole position at the same time.
The strands of the standard synchronous linear motor are connected according to FIG. 2. A diagram as shown in FIG. 3 results for the currents of the individual phases U, V, W depending on the travel displacement x. For the synchronous linear motor with a number of strands m=3, the result is the usual phase shift of Δφel=120° el. for the three phases. At the same time, a constant force is produced over the travel displacement x and: Fmax(x)=FNenn and Fmin(x)=FNenn.